A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. including part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at once, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
In order to monitor the lithographic process, it is desirable to measure parameters of the patterned substrate, for example the overlay error between successive layers formed in or on it. There are various techniques for making measurements of the microscopic structures formed in lithographic processes, including the use of scanning electron microscopes and various specialized tools. One form of specialized inspection tool is a scatterometer in which a beam of radiation is directed onto a target on the surface of the substrate and properties of the scattered or reflected beam are measured. By comparing the properties of the beam before and after it has been reflected or scattered by the substrate, the properties of the substrate can be determined. This can be done, for example, by comparing the reflected beam with data stored in a library of known measurements associated with known substrate properties. Two main types of scatterometer are known. Spectroscopic scatterometers direct a broadband radiation beam onto the substrate and measure the spectrum (intensity as a function of wavelength) of the radiation scattered into a particular narrow angular range. Angularly resolved scatterometers use a monochromatic radiation beam and measure the intensity of the scattered radiation as a function of angle.
One property of a substrate that can be measured using a scatterometer is overlay, that is the difference in position between two process layers in the substrate that ought to be exactly aligned. To measure overlay, a target, such as a grating, is printed in each of the layers whose relative overlay is to be measured at nominally the same position. The combined target is then inspected with a scatterometer and the mis-alignment of the two targets can be detected from the scatterometer spectrum. Overlay is measured in two directions, X and Y, so that if linear gratings are used two targets are required for each measurement site. Given that there may be 20 or 30 process layers in a complete device and multiple overlay measurement sites per target portion, while each target may be about 40 m by 40 m to accommodate the whole measurement spot, a substantial amount of space on the substrate is taken up by targets and hence not available for device structures.
It has therefore been proposed to use a two-dimensional grating, or checkerboard grating, which halves the amount of space required. However, it has been determined that there is cross-talk between the overlay in the X and Y directions as measured by a scatterometer. Such cross-talk complicates the computation of the measurement to be made and reduces its accuracy.